The conventional fluorine-based grease comprises perfluoropolyether as a base oil, a homopolymer [PTFE] of tetrafluoroethylene [TFE], a copolymer [FEP] of TFE with hexafluoropropylene, a copolymer [PFA] of TFE with perfluoroalkylvinylether, a copolymer [ETFE] of TFE with ethylene, etc. as a thickener, and a small proportion of various additives such as a rust preventive, etc. and is used under strict conditions requiring a low-temperature resistance, a high-temperature durability, an oxidation stability, a chemical resistance, etc.
However, the base oil and the thickener are both fluorine-containing polymers, and thus involve such problems as a high cost, less compatibility with materials to be lubricated such as resins, metals, rubber, etc., or failure to form necessary oil films for lubrication under the conditions such as high load, giving rise to abrasion, or making the friction coefficient so high that the torque transmission efficiency is lowered and the conventional fluorine-based grease has such problem as deterioration of the rust prevention and corrosion resistance.
To solve the problems, it has been so far proposed to use a mixture of fluorine-based grease with a non-fluorine-based grease, as is given in, for example, Patent Document 1, where the mixture is a grease comprising hydrogenated mineral oil and/or synthetic lubricating oil, fluoropolyether oil, and an organic or inorganic thickener in a ratio by weight of lubricating oil+fluoropolyether oil:thickener=97:3˜80:20, and a ratio by weight of lubricating oil:fluoropolyether oil=95:5˜60:40.
Patent Document 1: JP-A-7-268370
For the mixing to prepare the grease from such a base oil mixture, it is recommended to use a homogenizer such as Manto Galvin type homogenizer or a three-cylinder homogenizer (which can be presumed to be a three-partitioned cylinder block type homogenizer), while it is preferable for better homogeneity to make run number of the homogenizer treatment 2 or 3 times as large as that for the ordinary non-fluorine-based grease. However, as will be given in results of the following Comparative Examples, it is difficult to obtain a homogeneous grease mixture, even if the run number of the mixing treatment by the homogenizer is increased.
Patent Documents 2 and 3, both of which are filed by the present applicant, disclose a process for producing a lubricating grease composition comprising a non-fluorine-based grease and a fluorine-based grease by thorough kneading through three rolls or a high pressure homogenizer, where the three rolls and the high pressure homogenizers are regarded as equivalent kneading means, but no mention is made therein as to the run number of kneading at all.
Patent Document 2: JP-A-2003-96480
Patent Document 3: JP-A-2006-182923
Such grease mixture is less expensive than the single fluorine-based grease, and also has a distinguished abrasion resistance to the mating materials, but as a result of the mixing proportion of the fluorine-based base oil for forming the grease is limited from the viewpoint of its compatibility, the characteristic of the fluorine-based grease, that is, a good heat resistance, cannot be fully demonstrated. Furthermore, the proposed grease mixture has still such problems that any index of homogeneously dispersion of mutually incompatible base oils themselves is not shown therein and the individual base oils of the grease may be sometimes separated from each other, or the grease may be rapidly softened when exposed to a shearing force, depending on the degree of dispersion.